The disposal of hazardous chemical and/or radioactive waste (herein "waste") is desirable to minimize health and environmental risks. The hazard potential of such a waste is reduced by converting the contaminants (e.g., chemical and/or radionuclide toxic contaminants) into a reduced soluble, mobile or toxic form, i.e., stabilization. In one disposal method, the contaminants contained in the waste are stabilized by converting the waste materials to solidified waste by a solidification process wherein a solidifying agent, such as a hydraulic cement, is added to the waste to encapsulate the waste in a monolithic solid of high structural integrity (such as structural concrete material). The encapsulation may be of relatively fine solid waste particles (i.e., micro-encapsulation) or of a relatively large block or container of wastes (i.e., macroencapsulation). It is desirable that solidification mechanically bind the waste into the concrete sufficiently for the solidified waste to exhibit acceptable leachability, i.e, to pass one or more well-recognized leaching tests, such as those of The American Nuclear Society (ANS 16.1 Test), the State of California Waste Extraction Test (WET), and the United States Environmental Protection Agency (EPA) tests including the Extraction Procedure Toxicity Test (EPT), the Multiple Extraction Procedure (MEP), and the Toxicity Characterization Leaching Procedure (TCLP). Chemical and/or radionuclide toxic contaminant migration in the solidified waste is typically restricted (stabilized) by vastly decreasing the surface area exposed to leaching and/or by isolating the wastes within a relatively impervious capsule. One type of solidified waste contains an inorganic cementitious material such as a hydraulic Portland cement or a Portland cement plus other additives (such as lime, flyash and/or clay).
Precursors of solidified waste material contain a variety of waste-containing fillers and solidifying (cementing) agents, and form a hydraulic slurry when mixed with water. Waste-containing fillers are capable of being converted to a monolithic solid (such as concrete) after being combined with the solidifying agent. Examples of such fillers are silicious sludge obtained from silica-rich geothermal brine, Naturally Occurring Radioactive Materials (NORM), and tailings from mining operations. The hydraulic slurry, which can temporarily flow like a liquid or be pumpable (by ordinary concrete pumping means), is applied to the interior of a storage vessel, and permitted to cure (slowly hydrate or precipitate) into a rigid, hardened solidified waste that stabilizes the contaminants. Examples of storage vessels are subterranean formations such as caverns, fissures and cracks in the rocks or formation material, plugged or abandoned petroleum or geothermal wells (including fractured wells), abandoned mines, and molded forms such as blocks stored in landfills. The slurry may set or be cured "insitu" in the storage vessel.
During storage, some hydraulic (water-based) cements (e.g., Portland cement), fillers and, hence, monolithic solids made from the slurry, are subject to attack by external influences that may cause destabilization. When new components are used to produce stabilized solidified wastes, the primary considerations are that the components: 1) produce a workable slurry which can be applied to the interior of the storage vessel; 2) harden into a solidified waste which desirably attaches to the interior of the storage vessel; 3) resist long term changes effected by temperature, pressure, chemical (e.g., dissolution, corrosion, etc.) and mechanical (e.g., erosion, pulverization, etc.) attack, and the like; and 4) provide an effective barrier to attack of the surrounding material of the storage vessel. The solidified waste should also be rugged, safe, reliable, environmentally acceptable, and cost effective.
A common problem with some current monolithic (concrete) compositions useful for producing solidified waste is their propensity to crack. Such cracks allow attack of the encapsulated waste and the migration of the waste contaminants from the solidified waste. In subterranean storage vessels, geothermal applications can subject the solidified wastes to severe conditions. These conditions may crack brittle materials, particularly those experiencing tensile stresses. Many such materials are noted to shrink upon setting or curing. Shrinkage increases the tensile stress within the solidified waste and provides a void space within the container which can act as a migratory conduit. Cracking may even occur during preparation. An expansive or non-shrinking cement may be employed in the solidified waste material to maintain size and/or to generate compressive strength within the material in order to offset or circumvent shrinkage. Often such cements may not compact sufficiently to produce a desired matrix density of the hardened material or monolithic.
Additives are being sought to improve the compactibility of the solid slurry components, hence densifying the resulting solidified waste material. This will reduce shrinkage, thereby increasing the material's resistance to cracking and reduce porosity and permeability, consequently decreasing waste contaminant migration.